Often an employer will pay staff a higher wage for working on bank holidays, but this shouldn't be confused with a legal requirement, and there is no formula saying how much more you should be paid for working any given hours. Any extra payment is up to your employer to offer, and to state in your contract when you agree to take the job.
Where a union is recognised, extra pay or time off in lieu for working bank holidays will usually have been negotiated through a process known as collective bargaining.
If your employer has regularly offered double time in the past, such that it has become 'custom and practice' to receive it, and they are now refusing to pay, it could count as a breach of contract. If you think this might be the case, consult your union rep if you have one or seek legal advice.
This kind of issue is often best raised collectively, rather than sticking your head above the parapet on your own. It could be a good idea to speak to colleagues to see if they feel the same as you.
Unions are expert at negotiating improved terms such as better pay for bank holidays. If you are not a union member, browse our Union Finder tool to find out which union best suits your needs, and to discover more about organising to secure union recognition at your workplace.